15. 3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

public List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List<List<Integer>> res = new LinkedList<>();
    for (int i = 0; i < nums.length; i++) {
        if (i > 0 && nums[i] == nums[i - 1]) continue;
        int low = i + 1, high = nums.length - 1;
        while (low < high) {
            if (nums[i] + nums[low] + nums[high] == 0) {
                res.add(Arrays.asList(nums[i], nums[low], nums[high]));
                while (low < high && nums[low] == nums[low + 1]) low++;
                while (low < high && nums[high] == nums[high - 1]) high--;
                low++;
                high--;
            } else if (nums[i] + nums[low] + nums[high] > 0) high--;
            else low++;
        }
    }
    return res;
}

类似题目：18. 4Sum